drm/amdgpu: Use less generic enum definitions

alpha:allmodconfig fails to build as follows.

drivers/gpu/drm/amd/amdgpu/amdgpu.h:1006:2: error:
	expected identifier before '(' token
drivers/gpu/drm/amd/amdgpu/amdgpu.h:1011:28: error:
	'NGG_BUF_MAX' undeclared here

The problem is not really the enum definition of NGG_BUF_MAX but PARAM,
which happens to be defined differently for alpha and a couple of other
architectures.

Use less generic defines for NGG enums to solve the problem.

Fixes: bce23e00 ("drm/amdgpu: add NGG parameters")
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>